In general, a vehicular engine is provided with a starter having therein a dc motor and its switching mechanism. In many cases, this starter and an in-vehicle battery are connected together by a dedicated wiring, that is, a harness dedicated to this purpose. To restrain electric power loss, this harness is made as short as possible, and constructed to decrease the overall resistance including contact resistance as much as possible.
At the same time, there is known a vehicle having an idle stop function to stop an engine automatically at the time of a stop of the vehicle at a street crossing, for example, and to restart the engine automatically in accordance with brake release and/or accelerator pedal depression.
In the vehicle equipped with such an idle stop function, the battery voltage tends to drop frequently during operation of the vehicle due to inrush current of the starter motor, and the voltage drop might exert adverse influence on other electrical components and electronic equipment (an in-vehicle navigation system, for example). Therefore, there is proposed a technique to restrain the voltage drop due to the inrush current by inserting a resistor in series with the starter motor, as in a patent document 1.
However, the inrush current of the starter motor is very high at the level of several hundred amperes. Therefore, when an ordinary resistor is interposed in a circuit of the starter motor, the resistor tends to be heated to high temperatures and cause a problem concerning heat resisting property. Moreover, to avoid such heat generation, there is provided, in the patent document 1, a relay to short-circuit the resistor, and the system is arranged to supply current to the starter motor through the resistor only for a short time at an initial stage. However, in this case, the construction is complicated.
Furthermore, when the resistor is heated by repetition of the restart, the supply of current through the resistor even for a short period of time is unfeasible, and hence the system has to short-circuit the resistor, so that the system is unable to restrain the voltage drop.